This invention relates to the field of power amplifiers, and, more particularly, to microwave power amplifier harmonic frequency terminations.
Modern power amplifier architectures, such as balanced, push-pull, single ended and low noise power amplifiers, including microwave monolithic integrated circuit (MMIC) power amplifiers, have an increasing need for both linearity and power added efficiency.
Corporate feed networks, such as that shown in FIG. 1, are typically used to combine the outputs from many devices by terminating each device in the optimum load impedance for maximum power transfer. In corporate feed network 10 there are shown active device pairs 12a, 12b and 12c, 12d, and which may include other pairs as indicated by the dotted lines providing an even amount of active devices. The active devices, such as field effect transistors (FETS) or bi-polar junction transistors (BJTs), are fed by inputs 14a–14d. The sum of each of the active devices travel collectively through transmission lines, bends and tees 16 and are summed at common output terminal 18 to drive, for example, an antenna or transmission line.
Each of the active devices receive input signals over a frequency band spanning from a frequency range from f1 to f2, about a center frequency fo. Because the active devices provide power amplification, harmonics such as 2fo, 3fo, etc. are generated. Such harmonics can be problematic for other networks operating adjacent to the network. For larger power amplifier circuits that utilize a significant amount of space for output power combining and which also tend to be somewhat broadband in performance, it is difficult to adequately provide harmonic terminations because so much area is required for power combining.
Prior art approaches for harmonic terminations are typically narrow band and are usually accomplished using an RF choke and a large bypass capacitor as a low-pass filter coupled to the drain or collector of the amplifier stage. FIG. 2 shows a typical prior art low pass filter, which passes DC but not any higher frequencies. Included are RF choke 20 and large by-pass capacitor 22 to stop RF while allowing DC to pass. At higher frequencies, a quarter-wave transformer and bypass capacitor are sometimes used to supply the DC bias. The approaches work quite well but are somewhat narrowband (less than 20%) and do not lend itself to medium bandwidth applications such as is typical with the corporate feed scheme.
A need therefore exists for a harmonic termination approach for modern power amplifiers which provides both linearity and power added efficiency without interfering with corporate power combining or feed distribution schemes. Embodiments in accordance with the present invention provide practical solutions to meet such need.